As known, digital telecommunications systems implement, to exchange binary data, a predefined physical layer protocol which particularly aims at converting said binary data into a radiofrequency signal that can be transmitted in a predefined frequency band.
A physical layer protocol generally provides a succession of predefined steps.
In the case of a transfer of binary data from a terminal to a base station, the physical layer protocol provides, on the terminal side, steps applied to a binary data flow. These steps are, in particular, a modulation step, during which the binary data are converted into symbols (for example, BPSK, DBPSK, QPSK, 16QAM, etc.), and a frequency shift step, to obtain a radiofrequency signal centered on a predefined central frequency.
On the base station side, the physical layer protocol provides applying to a radiofrequency signal received from the terminal a processing inverse to that applied in transmit mode. Particularly, the radiofrequency signal should be frequency-shifted to obtain a baseband signal (that is, a signal centered on a substantially zero central frequency). The baseband signal, theoretically corresponding to a symbol sequence, is then demodulated to obtain binary data which, in the absence of errors, are equal to the binary data transmitted by the terminal.
The same steps are applied, in the case of a bidirectional telecommunications system, for a transfer of binary data from a base station to a terminal.
It should be noted that the physical layer protocol may provide many other steps, for example, an error correction coding step, an interlacing step, a filtering step, etc.
Further, such a physical layer protocol generally provides inserting control data intended to ease the inverse physical layer processing.
Indeed, the tasks to be performed in receive mode are by a much greater number than in transmit mode since it is generally necessary to detect whether a radiofrequency signal has been transmitted, to estimate the time of beginning of said radiofrequency signal (time synchronization) and the central frequency of said radiofrequency signal (frequency synchronization), to estimate the propagation channel in order to compensate for its effects, etc.
As a result, there is a large number of inverse processing operations to be performed in receive mode, which requires a high calculation power. This is all the more critical for base stations, which may have to simultaneously receive binary data from several terminals. Further, base stations perform other operations, relative to the processings of protocol layers which use the physical layer services (for example, MAC, TCP/IP, etc.).